Liverpool defender Trent Alexander-Arnold. Picture: Catherine Ivill/Getty Images
The right-back has been left at home for this evening's Group B fixture.
What has happened?
Alexander-Arnold trained with the Reds yesterday before they jetted off to Portugal.
However, Jurgen Klopp admitted during his pre-match press conference there were a couple of players whose fitness Liverpool needed to decide on.
Alexander-Arnold was one of those as he is suffering with a groin problem.
He’s been rested to avoid a sustained period on the sidelines.
Klopp said: "We have one or two who we have to make a decision about that - who will train here and who will come with us."
Will Alexander-Arnold be fit for Manchester City?
The Reds welcome their Premier League title rivals to Anfield on Sunday.
Klopp will want all of his key players available for the visit of Pep Guardiola's men.
But the Liverpool boss has confessed Alexander-Arnold is a major doubt.
Speaking ahead of the Porto game, Klopp said: “It didn’t look serious but in the end it was serious. He doesn’t look great for the City game.
"It's a muscle issue. He came in from training and we did a further assessment and it looked [more] serious. It’s a muscle, it’s a bit of tightness.”
Who has replaced him?
Alexander-Arnold missed the 3-0 defeat of Crystal Palace earlier this month because of illness.
It was James Milner who deputised and delivered a man-of-the-match performance despite not being a recognised full-back.
Milner again takes up duties in defence in Alexander-Arnold’s absence.
Is anyone else missing against Porto?
As expected, Thiago remains sidelined with a calf injury. He’ll also be absent against City.
Harvey Elliott (ankle) is a long-term casualty but Naby Keita is on the bench after missing Saturday’s 3-3 draw at Brentford with a foot problem.